1. Field of the Invention
This invention relates generally to color television receivers, and more particularly is directed to improvements in color television receivers of the beam index type.
2. Description of the Prior Art
In existing beam index color television receivers, a single electron beam scans the screen of a color cathode ray tube which, in its image or effective picture area, is provided with triads of red, green and blue color phosphor stripes extending vertically and being sequentially repeated in the horizontal scanning direction. The screen also has, on its inner surface, index phosphor stripes or elements which are spaced apart in the horizontal scanning direction in predetermined positional relation to the triads of color phosphor stripes. The index phosphor stripes emit radiant energy when excited by the scanning beam to cause a photodetector or the like to produce a corresponding index signal which is employed to control the gating or switching of the color signals for modulating the intensity or density of the electron beam. Thus, by employing the index signal, the electron beam can be density-modulated or have its beam current determined by the red, green and blue primary color signals at precisely the points where the single electron beam scans the color phosphor stripes which emit red, green and blue light, respectively.
In one conventional method of arranging the index phosphor stripes or elements, three index stripes are formed for every two sets or groups of red, green and blue color phosphor stripes so that the distance between adjacent index phosphor stripes is less than the pitch of each set or group of the color phosphor stripes. Although this conventional method has the advantage that the phase of the index signal is not displaced in response to the colors to be reproduced, it has the drawback that the index phosphor stripes are not located adjacent to known ones of the triads of color phosphor stripes. Accordingly, when a pulse having a triplet frequency is formed by multiplying the frequency of the index signal by three or, in the case of frequency division of the pulse in order to form three phase gating pulses adapted to gate the color signals for modulating the single electron beam, special circuitry must be employed for setting the mode of the gating pulses. The circuitry provides the respective gating pulses when the electron beam scans the color phosphor stripes in order to insure that the beam is properly modulated when it scans each color phosphor element or stripe.
One conventional method of setting the mode involves also arranging index phosphor elements on the run-in area located at one side of the effective picture or image area of the cathode ray tube and counting the index phosphor elements in the run-in area in order to set the mode, that is, phase and sequence of the three phase gating pulses at the beginning of each horizontal scan of the image area. However, this method has the possible drawback that if, during scanning of the image area, the video signal is at the black level so that no beam current flows, there will not be any index signal generated. This drawback is more fully explained in U.S. Patent Application Ser. No. 78,206, filed Sept. 24, 1979 and having a common assignee herewith, and which provides a means for preventing the picture tube from being subjected to this cut-off irrespective of the contents of the received video signal. However, the methodology and circuitry therein disclosed envisions positively establishing a minimum beam current by which the production of an adequate index signal is insured, irrespective of the contents of the received video signal when scanning the image area of the screen. This minimum beam current results in the black level of the video signal becoming higher or brighter, thereby undesirably reducing the contrast between the black or dark portions of the reproduced image and the color or relatively bright portions thereof.